1. Field of the Invention
This invention relates to a semiconductor processing facility for providing increased oxidation rates under standard conditions or for providing existing oxidation rates at reduced temperatures.
2. Description of the Prior Art
The procedure of thermally growing oxides on silicon is an important processing step in the manufacture of silicon semiconductor devices and several systems are known for performing such functions. One prior art system is composed of a quartz tube with open ends with a resistive heat element disposed about the quartz tube. In this prior art system, oxygen or steam is flowed through the open quartz tube whereby heated silicon members within the tube are oxidized to form the thermally grown oxide on the surface thereof. Quartz tubes of this type can be used in either a batch or continuous process. Oxidation takes place at ambient pressures which are essentially atmospheric.
The major limitation of this prior art system is the amount of time at elevated temperatures (typically 900.degree. to 1000.degree. C.) that is required to grow thermal oxides. For example, a 1 .mu.m thick field oxide would require approximately 4 hours to grow at 1000.degree. C., and approximately 12 hours to grow at 900.degree. C. in steam. While these thermal cycles are tolerated, it is known that both the time and temperature of oxidation processes must in general be minimized to optimize the yield and quality of finished semiconductor devices. In addition, oxidation time must also be minimized to increase manufacturing throughput and improve the efficiency of overall energy use in oxidation furnaces.
For these reasons, oxidations at elevated pressures have been explored, since it is known that the oxidation rate of silicon can be increased by operating at elevated pressure or, alternatively, the generating temperature required to achieve a given growth rate can be lowered if the pressure is elevated. In general, the oxidation time at a given temperature is inversely proportional to pressure; the oxidation temperature may also be reduced by approximately 30.degree. C. for each atmosphere of pressure increase while maintaining a given oxidation rate. For example, a 1 .mu.m thick field oxide grown at 900.degree. C. in steam would require only .about.2.5 hours of oxidation time at 5 atmospheres and only .about.1.25 hours of oxidation time at 10 atmospheres of pressure.
Experiments in oxide growth on silicon at elevated pressures have been carried out using a second prior art system which comprises a thin quartz tube with a surrounding metal jacket to provide support for the quartz tube and also to seal the quartz tube from the external environment, and a resistive heater around the quartz tube between the tube and the jacket. In this prior art system the pressure within the quartz tube and external to the tube between the tube and the steel jacket are approximately equal. This system therefore suffers the problem that contamination from the metal jacket travels into the quartz tube and presents contaminants to the silicon devices upon which fabrication steps are being performed. In this system, the metal jacket with the quartz tube inside is sealed and steam or oxygen in a nitrogen ambient is passed into the enclosure to a pressure of up to 25 atmospheres. The steam or other ambient passes through the open ends of the quartz tube so that the pressure on both sides of the quartz tube can be approximately the same. For this reason a thick quartz tube is not required. However, this high pressure system occupies a much larger space than oxidation furnaces operating at atmospheric pressures.
Accordingly, it is desired to provide a system for thermally oxidizing silicon which is capable of withstanding the higher pressures encountered to permit operation with decreased temperature and/or decreased time requirements, which is capable of continuous operation and which maintains the area wherein oxidation takes place substantially free of contaminants. Furthermore, the system should be capable of being fitted into a conventional atmospheric furnace enclosure, thereby substantially decreasing the amount of floor space required and hence minimizing cost.